Components of gas mixtures are frequently separated from the gas mixtures by PSA. Although PSA is generally more useful when the desired component is the least strongly adsorbed component, this gas separation technique can be successfully used when the desired component is more strongly adsorbed by the selected adsorbent than are the other components of the gas mixture. For example, carbon monoxide can be separated from gas mixtures containing, in addition to carbon monoxide, hydrogen, carbon dioxide, methane and nitrogen by means of cuprous ion-containing adsorbents. Such mixtures often occur in syngas, a hydrogen and carbon monoxide mixture produced in hydrocarbon reforming processes.
PSA processes are commonly carried out in elongate vessels having an inlet end and a nonadsorbed product outlet end and which are packed with a bed of particulate material which adsorbs one or more components of the gas mixture more strongly than it adsorbs one or more other components of the gas mixture. PSA processes include the basic steps of adsorption or production and adsorbent regeneration. During the adsorption step, the gas mixture to be separated is passed cocurrently through an adsorption vessel (in the direction from the inlet end towards the nonadsorbed product outlet end) at a selected adsorption pressure. The strongly adsorbed component(s) are adsorbed from the gas mixture as it passes through the vessel, and the nonadsorbed component(s) pass out of the vessel through the nonadsorbed product outlet. During the adsorbent regeneration step, the vessel is depressurized by releasing or withdrawing (evacuating) gas countercurrently (in the direction opposite the cocurrent direction) out of the vessel. The strongly adsorbed component(s) are removed from the vessel during the adsorbent regeneration step.
In addition to the basic steps, PSA processes generally have a number of additional steps. One important step is bed equalization, in which gas discharged from an adsorption vessel upon completion of the adsorption step (equalization-depressurization) is reintroduced in the vessel (or introduced into another vessel, in the case of multivessel systems) after completion of the adsorbent regeneration step (equalization-repressurization). During equalization, the gas can be removed from a vessel via its outlet end and reintroduced into the vessel via its outlet end (outlet-to-outlet equalization) or it can be removed from the vessel via its inlet end and reintroduced into the vessel via its inlet end (inlet-to-inlet equalization) or by other combinations of these. Conventional bed equalization serves two important purposes: it saves energy by using compressed gas released from an adsorption vessel after an adsorption step to partially repressurize the vessel for the adsorption step of the following cycle of the process, and it allows recovery of valuable partially fractionated gas component.
Another step that enhances the efficiency of adsorption processes is the purge or rinse step. This step generally assists in the regeneration of the adsorbent, and it can take place before, during, or after the countercurrent depressurization step. When the primary purpose of the PSA process is to recover nonadsorbed product, the purge is carried out by passing low pressure nonadsorbed product gas countercurrently through the adsorption vessel during or after the countercurrent depressurization step. When the primary product is the strongly adsorbed component of the mixture, the purge step is carried out by passing strongly adsorbed gas cocurrently through the vessel before, during or after countercurrent depressurization of the vessel. In any event, the purge step serves to remove additional undesired component from the adsorbent by increasing the partial pressure of the desired component in the vessel, thereby causing desorption of undesired component from the adsorbent, and by flushing undesired component from the vessel.
When a PSA process is practiced to recover a strongly adsorbed component from a gas mixture and the gas mixture contains more than one component that is preferentially adsorbed by the adsorbent, it is difficult to recover the desired component in high purity and at high yield. Improvements which enhance the purity and yield of strongly adsorbed products produced in PSA processes are continually sought. The present invention provides such an improvement.